Injury or damage to bones, discs, joints, and ligaments of the body are often a result of advancing age, trauma, tumor growth, or a disease process. In patients suffering from a degenerative disc disease, the injury often manifests itself as damage or degeneration of a spinal disc, the result of which can range from mild to severe chronic back pain. Intervertebral discs are fibrous cartilage pads that allow the spine to bend and serve as “shock” absorbers for the vertebrae, absorbing pressure delivered to the spinal column. Additionally, they maintain the proper anatomical separation between two adjacent vertebra. This separation is necessary for allowing both the afferent and efferent nerves to exit and enter, respectively, the spinal column.
To alleviate the pain caused by a ruptured or herniated disc, current treatment methods include a discectomy in which the affected intervertebral disc is removed. If desired, a disc prosthesis can be inserted between the vertebrae to fill the space left by the removed disc. Thereafter, the two adjacent vertebral bodies can be fused together in a process commonly referred to as spinal fusion. The disc prosthesis restores the angular relationship between the adjacent vertebrae to be fused, and provides the material for bone growth to occur between the two vertebral bodies.
A variety of these disc implants are known to exist. These artificial intervertebral discs can include fusion cages made from metals and/or synthetic materials. Many prostheses can also be fashioned from allograft bone that is harvested from portions of long bone including the femur, humerus, tibia, fibula, ulna and radius.
The success or failure of the fusion can often depend upon the type and properties of the prosthesis that is placed between the adjacent vertebral bodies. Obviously, the prosthesis must be sufficiently strong to withstand the loads encountered in the spine, it must be biocompatible, and it should permit the ingrowth of bone to complete the fusion. Perhaps more importantly, the prosthesis must have properties and geometries that permit the prosthesis to remain fixed in the desired position. Because of the continuous forces that act upon the vertebrae and especially the disc prosthesis in a dynamic environment like the human spinal column, the tendency of the prosthesis to migrate due to shifting, rotation or slippage poses a constant threat to the success of the bone fusion.